The ever growing bandwidth demands in communication systems continue to drive the need for higher-speed optical interconnection networks. To scale the interconnection interface bandwidth beyond, for example, 1 Tb/s, more bandwidth-efficient communication systems are likely needed to accommodate the bandwidth demands.
Optical coherent communications allow information to be encoded in the phase and/or frequency in addition to amplitude of an optical signal. Such systems allow for complex modulation in which a single symbol can encode two or more bits of information. Among various coherent modulation formats that are being developed, 5 bits per symbol (per polarization) 2-D coherent modulation formats appear to be the most promising formats. Since these formats allow a single laser to transmit, for example, about 800 Gb/s capacity by using ˜40 GHz optical and electrical component bandwidth, systems based on these formats offer the most likely success. However, currently available 5 bits per symbol coherent modulation formats are insufficient to accommodate the bandwidth demands due to various noise and tolerance issues with the formats.